1. Field of the Invention
The invention relates to an improved liquid light having a liquid core and tubular cladding made from a plastic material.
2. Description of the Prior Art
A liquid lightguide is known from German Patent Application P42 330 87 comprising a cylindrical tubular cladding consisting of a carbon fluoropolymer and comprising a liquid core which is surrounded by the core and comprises an optically transparent aqueous solution. The tubular cladding is coated on its interior surface by a thin layer of a completely amorphous copolymer, the latter being based on the combination of tetrafluorethylene with a perfluorated cyclic ether. The copolymer defining said internal layer is available from Dupont de Nemours and is marketed under the trade name Teflon.RTM. AF.
Teflon.RTM. AF is soluble in certain perfluorinated liquids in amounts of several percents by weight, suitable solvents being the Fluorinert.RTM. liquids FC 75, FC 77, or FC 40 made by 3M. The deposition of the AF-layer onto the internal surface of a carbon fluoropolymer tube such as Teflon.RTM. FEP is readily achieved by wetting the internal surface of the tube with the AF containing solution and successively by evaporating the solvent by passing air therethrough or by applying reduced pressure. The thickness of the layer is only a few microns which--in the case of a Teflon.RTM. FEP tube--is sufficient for total index reflection of visible and ultraviolet radiation. The advantages of a Teflon.RTM. AF totally reflecting layer reside in its extremely low refractive index which is in the range of from 1.29 to 1.32, its absolute transparency which is comparable to fused silica glass, and its chemical inertness. Due to their photochemical stability in the ultraviolet region, aqueous salt solutions are preferred as lightguides core liquids, such as chlorides or phosphates described in DE 24 06 424 C2 and P 40 19 363.5 which have been successfully applied for more than twenty years. These liquids, such as CaCl.sub.2 or NaH.sub.2 PO.sub.4 dissolved in H.sub.2 O, should have a refractive index greater than that of the totally reflecting Teflon.RTM. AF layer; owing to the extremely low refractive index of Teflon.RTM. AF layer, liquids may be used having refractive indices of as low as n=1.36. The optical aperture angle 2.alpha. should be at least 50.degree., wherein .alpha. may be calculated by the simple formula: EQU sin .alpha.=n.sup.2.sub.core -n.sup.2.sub.layer +L
Liquid lightguides having a core liquid consisting of an aqueous phosphate solution, such as NaH.sub.2 PO.sub.4 in water, and having a particularly high photochemical resistance in the UVB and UVC radiation spectral range (see P 40 14 363.5) can only be achieved by applying a totally reflecting layer having a refractive index in the range of about n=1.31, such as Teflon.RTM. AF 1600, since such solutions--due to precipitation of salt at lower temperatures--do not allow a refractive index greater than n=1.38.
A further advantage of Teflon.RTM. AF is given by its good adhesion on substrates consisting of carbon fluoropolymers, particularly after subjecting the layer to temperature baking where it is heated to temperatures slightly higher than its glass transition temperature.
A more serious draw back of Teflon.RTM. AF is its extreme high price of USD 10.00 per gram, which remarkably influences the productions costs of liquid lightguides. A further disadvantage of Teflon.RTM. AF is given by the fact that the material adheres only well on substrate materials that--as Teflon.RTM. AF itself--likewise consist of carbon fluoropolymers, and secondly that thin layers of Teflon.RTM. AF do not have elastic properties.
However this fact was acceptable in that liquid lightguides prevailing on the market during the last twenty years have been consisting of Teflon.RTM. FEP tubings filled with an aqueous CaCl.sub.2 solution, so that an increase in aperture could be easily achieved by internally coating these Teflon.RTM. AF tubings with Teflon.RTM. AF without the necessity of developing new substrate-layer combinations.
However, it would be desirable if other silicon or polyurethane based tubing material could likewise be internally coated, allowing the production of new lightguides with new properties, such as enhanced flexibility. Moreover, internal coating of glass tubes would be desirable to allow for the production of light conducting optical cells.
In EP 02 46 552.B1, Fitz describes a carbon fluoropolymer practically completely amorphous and soluble in certain acetone-like liquids, recommending its use for internally coating of liquid core lightguides. This material--a terpolymer consisting of the components TFE (tetrafluoroethylene), HFP (hexafluoropropylene), and VDF (vinylidene fluoride)--is highly transparent and does not only stick on plastic substrate materials consisting of carbon fluoropolymeres; however, the material has the disadvantage of having a refractive of n=1.36 which is much higher than that of Teflon.RTM. AF and of being hardly applicable to liquid lightguides transmitting in the ultraviolet spectral range.